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    I hear it all the time, "if you skip breakfast or lunch you will go into STARVATION MODE". What REALLY happens to your body, during SHORT TERM FASTING (0-72hours) , and LONG TERM FAMINE/STARVATION ( >72 hours).
    The average human body has about 6 hours of stored Glycogen (sugar) in its skeletal muscle and liver. When this runs out, the body switches to converting fat into Glycerol and free fatty acids. The Glycerol is a substrate that is converted by the liver into Glucose, this process is known as "Gluconeogenesis". This Glucose is then used to fuel the brain and other organs. The Free Fatty Acids can be used as a direct fuel source by most of the other tissues in the body, except the brain because they cannot cross the blood-brain barrier. This is the source of fuel for the first 2-3 days. The reason that most of the energy doesn't come from muscle breakdown during this time is because during the first 3-3.5 days is, the body releases Human Growth Hormone which has a "protein sparing' effect on skeletal muscles. Free Fatty Acids are the primary fuel source for the first 2-3 days. This means that the body tissues are using free fatty acids as fuel and they "shut down" their use of glucose, except the brain. Our brains require about 120 g of glucose (3 cans of soda) per day to function. At this rate the brain with quickly use up the body's remaining stores of glucose. The body has a "Plan B" however,"Ketone Bodies". Ketone Bodies are "short-chain derivatives" of free fatty acids. These shorter molecules are able to cross the blood-brain barrier to fuel the brain. This process known as "KETOSIS", not to be confused with "DIABETIC KETOACIDOSIS", occurs after the first 2-3 days, they are produced in the liver from the free fatty acids in the blood. This use of ketones cuts the brains requirement for glucose. After 3 days of Fasting, your brain gets about 30% of its energy from ketone bodies. After 4 days this goes up to 70%. This reduces the 120 g a day, down to about 30 g a day. Of this remaining 30 grams, the liver can produce around 20 g a day from free fatty acids but the other 10 g come from another source.
    The last source available is our bodies Protein / Amino Acids. After several days of fasting, all cells in the body begin to break down protein into Amino Acids, these are then sent to the liver for gluconeogenesis in order to provide the last 10 g required. Only at this point, beyond 3.5 days, does the body start to break down its skeletal muscles and later, organ proteins in the heart, etc. About 2-3 g of protein have to be broken down just to create 1 g of glucose, however, this process will slow down the longer the deprivation of food continues, to conserve precious muscle and protein. TRUE STARVATION only ensues when the fat reserves have been completely exhausted and protein is the ONLY fuel source available. If starvation reaches this point, the body starts using the protein of the heart and eventually leads to cardiac failure and death. In leaner people, the fat reserves are depleted much earlier, therefore death occurs earlier.
    Below is a simplified timeline of the starvation process;

    0 Hours: Glucose still used as primary fuel.

    0-6 Hours: Glycogen stores are broken down to provide glucose for fuel.

    6-72 Hours: Glycogen stores in muscle and liver are used up and the body breaks down fat into free fatty acids to create glucose and Ketone bodies for fuel.

    >72 Hours: Body cells break down Protein from muscle and eventually vital organs to create glucose from amino acids to keep the brain alive.

    I hope this helps to put to rest the dreaded "STARVATION MODE" myth everyone is so worried about when they skip meals or eat just once a day. you can read the difference between STARVATION vs. FASTING in a later post. Thank you and STAY STRONG, STAY HUNGRY !


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